Dr. Robert "Rack" Hansen, a veterinarian in rural Verde Valley, Arizona, receives an urgent call from a local farmer, Walter Colby. Colby is upset because his prize calf has become sick for no apparent reason, and the animal is brought in to Hansen's laboratory. Hansen examines the calf, which dies shortly afterward. Hansen tells Colby he cannot explain what made the animal so ill so quickly, but takes samples of the calf's blood to a university lab in Flagstaff, Arizona.